Sec 4 Physics Electromagnetic Induction topical questions

State the condition needed for an e.m.f. to be induced in a coil.

State two changes that would increase the size of the induced e.m.f. when a magnet moves into a coil.

State Lenz’s law.

Write down the equation relating the primary and secondary voltages of a transformer to the numbers of turns.

Name the device that keeps a rotating generator coil connected to the outside circuit while allowing its output to remain alternating.

A coil is connected to a centre-zero galvanometer. A bar magnet is pushed into the coil, held still, then pulled out quickly.

1. (a)Describe the galvanometer readings at each of the three stages.[3]
2. (b)Explain each reading using flux linkage.[3]

A step-up transformer has 100 primary turns and 500 secondary turns. The primary is connected to a 12 V a.c. supply.

1. (a)Calculate the secondary e.m.f.[2]
2. (b)The secondary supplies 0.50 A. Assuming the transformer is ideal, calculate the primary current.[2]
3. (c)Explain why a d.c. supply of 12 V would not keep the secondary working.[1]

A bicycle dynamo (a small generator) lights a lamp as the wheel turns. The rider notices the lamp is dim when cycling slowly and bright at speed, and the lamp flickers at a faster rate at speed.

1. (a)Explain why the lamp is brighter at speed.[3]
2. (b)Explain why the flicker (frequency) is faster at speed.[2]

The laminated soft-iron core of a mains transformer becomes warm after an hour of use.

1. (a)Name the loss process in the core and explain how lamination reduces it.[3]
2. (b)Name one other loss in a transformer and state how it is reduced.[2]

A small power plant sends 48 kW along a cable of total resistance 0.50 Ω to a village.

1. (a)Calculate the current and the cable power loss if the power is sent at 240 V.[3]
2. (b)Calculate the current and the cable power loss if the power is sent at 12 kV.[3]
3. (c)Express the fractional loss in each case and state the conclusion.[2]

A magnet is dropped through a horizontal copper ring, then through a copper ring with a small gap cut in it.

1. (a)Describe and explain the magnet’s motion through the complete ring.[4]
2. (b)State and explain how the motion differs through the gapped ring.[3]

A phone charger contains a transformer stepping 230 V to 5.0 V. Its primary has 1150 turns and it supplies 3.0 A to a phone on its secondary.

1. (a)Calculate the number of secondary turns.[2]
2. (b)Assuming an ideal transformer, calculate the primary current.[3]
3. (c)In reality the charger draws slightly more primary current than calculated. Suggest why.[2]

A student connects two coils, A and B, on the same iron core. Coil A is connected to a battery and a switch; coil B to a voltmeter. The student closes the switch, holds it closed, then opens it.

1. (a)Describe the voltmeter readings during each stage and explain them.[5]
2. (b)State what would be observed if the battery were replaced by a low-voltage a.c. supply.[2]
3. (c)Name the practical device that works exactly this way.[1]